Assigned Requests and Unassigned Requests

When a request first comes in from
an HTTP client to the load balancer, it is a request for a new session. A request
for a new session is called an unassigned request. The load balancer
routes this request to an application server instance in the cluster according to
a round-robin algorithm.

Once a session is created on an application server instance, the load balancer
routes all subsequent requests for this session only to that particular instance.
A request for an existing session is called an assigned or a
sticky request.

HTTP Load Balancing Algorithm

The Sun Java System Application Server load balancer uses
a sticky round robin algorithm to load balance incoming HTTP
and HTTPS requests. All requests for a given session are sent to the same application
server instance. With a sticky load balancer, the session data is cached on a single
application server rather than being distributed to all instances in a cluster.

When a new HTTP request
is sent to the load balancer plug-in, it’s forwarded to an application server
instance based on a simple round robin scheme. Subsequently, this request is “stuck”
to this particular application server instance, either by using cookies, or explicit
URL rewriting.

From the sticky information, the load balancer plug-in first determines the
instance to which the request was previously forwarded. If that instance is found
to be healthy, the load balancer plug-in forwards the request to that specific application
server instance. Therefore, all requests for a given session are sent to the same
application server instance.

The load balancer plug-in uses the following methods to determine session stickiness:

Cookie Method : the load balancer plug-in uses
a separate cookie to record the route information. The HTTP client must support cookies
to use the cookie based method.

Explicit URL Rewriting: the sticky
information is appended to the URL. This method works even if the HTTP client does
not support cookies.

Sample Applications

The following directories contain sample applications that demonstrate load
balancing and failover:

Prerequisites for Setting Up Load Balancing

Before configuring your load balancer, you must:

Install a web server.

Install the load balancer plug-in.

For information on
the installation procedure, see the Sun Java System Application Server
Installation Guide (if using the stand-alone Application Server) or the
Sun Java Enterprise System Installation Guide (if using Java
Enterprise System).

Using Clustered Server Instances

The most common way to deploy the load balancer is with a cluster or clusters
of server instances. By default all the instances in a cluster have the same configuration
and the same applications deployed to them. The load balancer distributes the workload
between the server instances and requests fail over from an unhealthy instance to
a healthy one. If you’ve configured HTTP session persistence, session information
persists when the request is failed over.

If you have multiple clusters, requests are only load balanced and failed over
between the instances in a single cluster. Use multiple clusters in a load balancer
to easily enable rolling upgrades of applications. For more information, see Upgrading Applications Without Loss of Availability.

Using a Single, Stand-Alone Instance with Load Balancer Used
as a Reverse-Proxy Plug-in

You can also configure your load balancer to use stand-alone server instance
instead of a cluster. This configuration results in the load balancer plug-in working
as a reverse-proxy plug-in
(sometimes called a pass-through plug-in). When the web server receives requests for
applications enabled in the load balancer, it forwards the requests directly to the
Application Server.

Use the same procedures to configure the load balancer for a pass-through plug-in
as you use to configure it for a cluster of server instances.

Using Multiple Stand-Alone Instances

It is also possible to configure your load balancer to use multiple stand-alone
instances, and load balance and fail-over requests between them. However, in this
configuration, you must manually ensure that the stand-alone instances have homogenous
environments and the same applications deployed to them. Because clusters automatically
maintain a homogenous environment, for most situations it is better and easier to
use clusters.

Procedure to Set Up Load Balancing

Use
the asadmin tool to configure load balancing in your environment.
For more information on the asadmin commands used in these steps,
see Configuring the Load Balancer

To Set Up Load Balancing

Create a load balancer configuration using the asadmin command create-http-lb-config.

Add a reference to a cluster or stand-alone server instance for the load
balancer to manage using asadmincreate-http-lb-ref.

If you created the load balancer configuration with a target, and that
target is the only cluster or stand-alone server instance the load balancer references,
skip this step.

Enable the cluster or stand-alone server instance referenced by the load
balancer using asadmin enable-http-lb-server.

These applications must already be deployed and enabled for
use on the clusters or stand-alone instances that the load balancer references. Enabling
for load balancing is a separate step from enabling them for use.

Create a health checker using asadmincreate-health-checker.

The health checker monitors unhealthy server instances so
that when they become healthy again, the load balancer can send new requests to them.

In the above code, lbplugin is a name that uniquely identifies
the Object, and web_server_install_dir/web_server_instance/config/loadbalancer.xml is the location of the XML configuration file for the virtual server on
which the load balancer is configured to run.

Using Apache Web Server

To use Apache Web Server, you must perform certain configuration steps before
installing the load balancer plug-in. The load balancer plug-in installation also
makes additional modifications to the Apache Web Server. After the plug-in is installed,
you must perform additional configuration steps.

Note –

On Apache 1.3, when more than one Apache child processes runs, each process
has its own load balancing round robin sequence. For example, if there are two Apache
child processes running, and the load balancer plug-in load balances on to two application
server instances, the first request is sent to instance #1 and the second request
is also sent to instance #1. The third request is sent to instance #2 and the fourth
request is sent to instance #2 again. This pattern is repeated (instance1, instance1,
instance2, instance2, etc.)This behavior is different from what you might expect,
that is, instance1, instance2, instance1, instance2, etc. In Sun Java System Application
Server, the load balancer plug-in for Apache instantiates a load balancer instance
for each Apache process, creating an independent load balancing sequence.

Apache 2.0 has multithreaded behavior if compiled with the --with-mpm=worker option.

On the Linux platform, install Sun Java System Application Server on the
same machine.

On the Solaris operating system, ensure that gcc version
3.3 and make are in the PATH, and flex is installed.

On the Solaris 10 operating system, before running make for
OpenSSL, run mkheaders, located in /usr/local/lib/gcc-lib/sparc-sun-solaris2.9/3.3/install-tools on Solaris SPARC or /usr/local/lib/gcc-lib/i386-pc-solaris2.9/3.3/install-tools on Solaris x86.

If you are using gcc on Red Hat Enterprise Linux
Advanced Server 2.1, the version must be later than gcc 3.0.

Note –

To use C compiler other than gcc, set the path of the
C compiler and make utility in the PATH environment variable. For example, with the sh shell: export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:appserver_installdir/lib

On the Linux platform, install Sun Java System Application Server on the
same machine.

On the Solaris operating system, ensure that gcc version
3.3 and make are in the PATH, and flex is installed.

On the Solaris 10 operating system, before running make for
OpenSSL, run mkheaders, located in /usr/local/lib/gcc-lib/sparc-sun-solaris2.9/3.3/install-tools on Solaris SPARC or /usr/local/lib/gcc-lib/i386-pc-solaris2.9/3.3/install-tools on Solaris x86.

If you are using gcc on Red Hat Enterprise Linux
Advanced Server 2.1, the version must be later than gcc 3.0.

Note –

To use a C compiler other than gcc, set the path of
the C compiler and make utility in the PATH environment variable. For example, with
the sh shell:export LD_LIBRARY_PATH=app_server_install_dir/lib:$LD_LIBRARY_PATH.

Configuration before Installing the Load Balancer Plug-in

Before installing the load balancer plug-in for Apache,
install the Apache Web Server. The Apache source must be compiled and built to run
with SSL. This section describes the minimum requirements and high-level steps needed
to successfully compile Apache Web Server to run the load balancer plug-in. These
requirements and steps only apply to the Solaris and Linux versions of the software.
For information on the Windows version of Apache, see the Apache web site.

To Install SSL-aware Apache

Before You Begin

You must have already downloaded and uncompressed the Apache software.

Download and unpack the OpenSSL source.

Compile and build OpenSSL.

This step is not required on the
Linux platform if OpenSSL 0.9.7.e is installed.

In the Environment Variables window, under System Variables, click New. Enter Variable
name of NSPR_NATIVE_THREADS_ONLY and Variable value of 1.

Restart the machine.

Modifications to Microsoft IIS

To configure Microsoft Internet Information
Services (IIS) to use the load balancer plug-in, modify certain properties in Windows
Internet Services Manager. The Internet Services Manager is located in the Administrative
Tools folder in the Control Panel folder.

Make these modifications after installing the Sun Java System Application Server.

To Configure Microsoft IIS to use the Load Balancer Plug-in

Open the Internet Services Manager.

Select the web site for which you want to enable the plug-in.

This web site is typically named the Default Web Site.

Right click on the web site and select Properties to open the Properties
notebook.

Add a new ISAPI filter, following these steps:

Open the ISAPI Filters tab.

Click Add.

In the Filter Name field, enter Application Server

In the Executable field, type C:\Inetpub\wwwroot\sun-passthrough\sun-passthrough.dll

Click OK, and close the Properties notebook.

Create and configure a new virtual directory:

Right click on the default web site, select New, and then Virtual Directory.

The Virtual Directory Creation Wizard opens.

In the Alias field, type sun-passthrough .

In the Directory field, type C:\Inetpub\wwwroot\sun-passthrough.

Check the Execute Permission checkbox.

Leave all other permission-related
check boxes are left unchecked.

Click Finish.

Add the path of sun-passthrough.dll file and application_server_install_dir/bin to the system’s PATH environment variable.

Restart the machine.

Stop and start the web server for the new settings to take effect.

To stop the web server, right click on the web site and select Stop . To start the web server, right click on the web site and select Start.

Type the following in a web browser to access the
web application context root: http://webserver_name/web_application, where webserver_name is
the host name or IP address of the web server and web_application is
the context root that you listed in the C:\Inetpub\wwwroot\sun-passthrough\sun-passthrough.properties file.

Automatically configured Sun-passthrough properties

The installer automatically configures the following properties in sun-passthrough.properties. You can change the default values.

Property

Definition

Default Value

lb-config-file

Path to the load balancer configuration file

IIS_www_root\sun-passthrough\loadbalancer.xml

log-file

Path to the load balancer log file

IIS_www_root\sun-passthrough\lb.log

log-level

Log level for the web server

INFO

Configuring Multiple Web Server Instances

The Sun Java System Application
Server installer does not allow the installation of multiple load balancer plug-ins
on a single machine. To have multiple web servers with the load balancer plug-in on
a single machine, in either a single cluster or multiple clusters, a few manual steps
are required to configure the load balancer plug-in.

To Configure Multiple Web Server Instances

Configure the new web server instance to use the load balancer plug-in.

Creating an HTTP Load Balancer Configuration

Create a load balancer configuration using the asadmin command
create-http-lb-config. Creating an HTTP Load Balancer Configuration describes the parameters. For more information
see the documentation for create-http-lb-config, delete-http-lb-config, and list-http-lb-configs.

Table 4–1 Load Balancer Configuration Parameters

Parameter

Description

response timeout

Time in seconds within which a server instance must return a response. If no
response is received within the time period, the server is considered unhealthy. The
default is 60.

HTTPS
routing

Whether HTTPS requests to the load balancer result in HTTPS or HTTP requests
to the server instance. For more information, see Configuring HTTPS Routing.

reload interval

Interval between checks for changes to the load balancer configuration file
loadbalancer.xml. When the check detects changes, the configuration
file is reloaded. A value of 0 disables reloading.For more information, see Enabling Dynamic Reconfiguration

monitor

Whether monitoring is enabled for the load balancer.

routecookie

Name of the cookie the load balancer plug-in uses to record the route information.
The HTTP client must support cookies. If your browser is set to ask before storing
a cookie, the name of the cookie is JROUTE.

target

Target for the load balancer configuration. If you specify
a target, it is the same as adding a reference to it. Targets can be clusters or stand-alone
instances.

Creating an HTTP Load Balancer Reference

When
you create a reference in the load balancer to a stand-alone server or cluster, the
server or cluster is added to the list of target servers and clusters the load balancer
controls. The referenced server or cluster still needs to be enabled (using enable-http-lb-server) before requests to it are load balanced. If you created
the load balancer configuration with a target, that target is already added as a reference.

Create a reference using create-http-lb-ref. You must supply
the load balancer configuration name and the target server instance or cluster.

To delete
a reference, use delete-http-lb-ref. Before you can delete a reference,
the referenced server or cluster must be disabled using disable-http-lb-server .

For more information, see the documentation for create-http-lb-ref and delete-http-lb-ref.

Enabling Server Instances for Load Balancing

After creating a reference to the server instance or
cluster, enable the server instance or cluster using enable-http-lb-server . If you used a server instance or cluster as the target when you created
the load balancer configuration, you must enable it.

For more information, see the documentation for enable-http-lb-server .

Enabling Applications for Load Balancing

All servers managed by a load balancer must have
homogenous configurations, including the same set of applications deployed to them.
Once an application is deployed and enabled for access (this happens during or after
the deployment step) you must enable it for load balancing. If an application is not
enabled for load balancing, requests to it are not load balanced and failed over,
even if requests to the servers the application is deployed to are load balanced and
failed over.

When enabling the application, specify the application name and target. If the
load balancer manages multiple targets (for example, two clusters), enable the application
on all targets.

For more information, see the online help for enable-http-lb-application .

If you deploy a new application, you must also enable it for load balancing
and export the load balancer configuration again.

Creating the HTTP Health Checker

The load
balancer’s health checker periodically checks all the configured Application Server instances
that are marked as unhealthy.
A health checker is not required, but if no health checker exists, or if the health
checker is disabled, the periodic health check of unhealthy instances is not performed.

The load balancer’s health check mechanism communicates with the application
server instance using HTTP. The health checker sends an HTTP request to the URL specified
and waits for a response. A status code in the HTTP response header between 100 and
500 means the instance is healthy.

Creating a Health Checker

To create the health checker, use the asadmincreate-http-health-checke r command. Specify the following parameters:

Table 4–2 Health Checker Parameters

Parameter

Description

Default

url

Specifies the listener’s URL that the load balancer checks to determine
its state of health.

“/”

interval

Specifies the interval in seconds at which health checks of instances occur.
Specifying 0 disables the health checker.

30 seconds

timeout

Specifies the timeout interval in seconds within which a response must be obtained
for a listener to be considered healthy.

10 seconds

If an application server instance is marked as unhealthy, the health checker
polls the unhealthy instances to determine if the instance has become healthy. The
health checker uses the specified URL to check all unhealthy application server instances
to determine if they have returned to the healthy state.

If the health checker finds that an unhealthy instance has become healthy, that
instance is added to the list of healthy instances.

For more information see the documentation for create-http-health-checker and delete-http-health-checker.

Additional Health Check Properties for Healthy Instances

The health checker created by create-http-health-checker only
checks unhealthy instances. To periodically check healthy instances set some additional
properties in your exported loadbalancer.xml file.

Note –

These properties can only be set by manually editing loadbalancer.xmlafter you’ve exported it. There is no equivalent
asadmin command to use.

To check healthy instances, set the following properties:

Table 4–3 Health-checker Manual Properties

Property

Definition

active-healthcheck-enabled

True/false flag indicating whether to ping healthy server instances to determine
whether they are healthy. To ping server instances, set the flag to true.

number-healthcheck-retries

Specifies how many times the load balancer’s health checker pings an unresponsive
server instance before marking it unhealthy. Valid range is between 1 and 1000. A
default value to set is 3.

If you add these properties, then subsequently edit and export the loadbalancer.xml file again, you must add these properties to the file again,
since the newly exported configuration won’t contain them.

Exporting the Load Balancer Configuration File

The load balancer plug-in shipped with Sun Java System Application Server uses
a configuration file called loadbalancer.xml. Use the asadmin tool to create a load balancer configuration in the domain.xml file. After configuring the load balancing environment, export it to a
file.

To export the load balancer configuration

Export a loadbalancer.xml file using the asadmin command export-http-lb-config.

Export the loadbalancer.xml file for a particular load balancer configuration. You can specify a path
and a different file name. If you don’t specify a file name, the file is named loadbalancer.xml.load_balancer_config_name.
If you don’t specify a path, the file is created in the application_server_install_dir/domains/domain_name/generated directory.

To specify a path on Windows, use quotes
around the path. For example, "c:\sun\AppServer\loadbalancer.xml" .

For example, for the Sun Java System Web Server,
that location might be web_server_root/config .

The load balancer configuration file in the web server’s
configuration directory must be named loadbalancer.xml. If your
file has a different name, such as loadbalancer.xml.load_balancer_config_name, you must rename it.

Changing the Load Balancer Configuration

If you change a load balancer configuration by creating or
deleting references to servers, deploying new applications, enabling or disabling
servers or applications, and so on, export the load balancer configuration file again
and copy it to the web server’s config directory. For more
information, see Exporting the Load Balancer Configuration File

The load balancer plug-in checks for an updated configuration periodically based
on the reload interval specified in the load balancer configuration. After the specified
amount of time, if the load balancer discovers a new configuration file, it starts
using that configuration.

Enabling Dynamic Reconfiguration

With dynamic reconfiguration, the load balancer plug-in periodically
checks for an updated configuration.

To enable dynamic reconfiguration:

When creating a load balancer configuration, use the --reloadinterval option with asadmin create-http-lb-config .

This option sets the amount of time between checks for changes to the load balancer
configuration file loadbalancer.xml. A value of 0 disables dynamic
reconfiguration. By default, dynamic reconfiguration is enabled, with a reload interval
of 60 seconds.

If you have previously disabled it, or to change the reload interval,
use the asadmin set command.

After changing the reload
interval, export the load balancer configuration file again and copy it to the web
server’s config directory, then restart the web server.

Note –

If the load balancer encounters a hard disk read error while attempting
to reconfigure itself, then it uses the configuration that is currently in memory.
The load balancer also ensures that the modified configuration data is compliant with
the DTD before over writing the existing configuration.

If a disk read
error is encountered, a warning message is logged to the web server’s error
log file.

The error log for Sun Java System Web Server’ is at: web_server_install_dir/webserver_instance/logs/.

Disabling (Quiescing) a Server Instance or Cluster

Before stopping an application server for any reason,
you want the instance to complete serving requests. The process of gracefully disabling
a server instance or cluster is called quiescing.

The load balancer uses the following policy for quiescing application server
instances:

If an instance (either stand-alone or part of a cluster) is disabled,
and the timeout has not expired, sticky requests continue to be delivered to that
instance. New requests, however, are not sent to the disabled instance.

When the timeout expires, the instance is disabled. All open connections
from the load balancer to the instance are closed. The load balancer does not send
any requests to this instance, even if all sessions sticking to this instance were
not invalidated. Instead, the load balancer fails over sticky requests to another
healthy instance.

Disabling (Quiescing) an Application

Before you undeploy a web application, you want to the application to
complete serving requests. The process of gracefully disabling an application is called
quiescing. When you quiesce an application, you specify a timeout period. Based on
the timeout period, the load balancer uses the following policy for quiescing applications:

If the timeout has not expired, the load balancer does not forward
new requests to the application, but returns them to the web server. However, the
load balancer continues to forward sticky requests until the timeout expires.

When the timeout expires, the load balancer does not accept any requests
for the application, including sticky requests.

When you disable an application
from every server instance or cluster the load balancer references, the users of the
disabled application experience loss of service until the application is enabled again.
If you disable the application from one server instance or cluster while keeping it
enabled in another server instance or cluster, users can still access the application.

Configuring HTTP and HTTPS Failover

The load balancer plug-in fails over HTTP/HTTPS sessions to another application
server instance if the original application server instance to which the session was
connected becomes unavailable. This section describes how to configure the load balancer
plug-in to enable HTTP/HTTPS routing and session failover.

HTTPS Routing

The HTTP Secure (HTTPS) protocol uses Secure Sockets Layer (SSL) to provide
encryption an decryption of HTTP requests for secure communication. For HTTPS routing
to work, one or more HTTPS listeners must be configured.

The load balancer
plug-in routes all incoming HTTP or HTTPS requests to application server instances.
However, if HTTPS routing is enabled, an HTTPS request will be forwarded by the load
balancer plug-in to the application server using an HTTPS port only. HTTPS routing
is performed on both new and sticky requests.

If an HTTPS request is received and no session is in progress, then the load
balancer plug-in selects an available application server instance with a configured
HTTPS port, and forwards the request to that instance.

In an ongoing HTTP session, if a new HTTPS request for the same session is received,
then the session and sticky information saved during the HTTP session is used to route
the HTTPS request. The new HTTPS request is routed to the same server where the last
HTTP request was served, but on the HTTPS port.

Configuring HTTPS Routing

The httpsrouting option of the create-http-lb-config command controls whether HTTPS routing is turned on or off for all the
application servers that are participating in load balancing. If this option is set
to false, all HTTP and HTTPS requests are forwarded as HTTP. Set it to true when creating
a new load balancer configuration, or change it later using the asadmin set command.

Note –

If https-routing is set to true,
and a new or a sticky request comes in where there are no healthy HTTPS listeners
in the cluster, then that request generates an error.

Known Issues

The Load Balancer has the following limitations with HTTP/HTTPS request processing.

If a session uses a combination of HTTP and HTTPS requests, then the
first request must be an HTTP Request. If the first request is an HTTPS request, it
cannot be followed by an HTTP request. This is because the cookie associated with
the HTTPS session is not returned by the browser. The browser interprets the two different
protocols as two different servers, and initiates a new session.

This
limitation is valid only if httpsrouting is set to true .

If a session has a combination of HTTP and HTTPS requests, then the
application server instance must be configured with both HTTP and HTTPS listeners.

This limitation is valid only if httpsrouting is set
to true.

If a session has a combination of HTTP and HTTPS requests, then the
application server instance must be configured with HTTP and HTTPS listeners that
use standard port numbers, that is, 80 for HTTP, and 443 for HTTPS. This limitation
applies regardless of the value set for httpsrouting.

Configuring Idempotent URLs

An idempotent request is one that does not cause any change
or inconsistency in an application when retried. In HTTP, some methods (such as GET)
are idempotent, while other methods (such as POST) are not. Retrying an idempotent
URL must not cause values to change on the server or in the database. The only difference
is a change in the response received by the user.

Examples of idempotent requests include search engine queries and database queries.
The underlying principle is that the retry does not cause an update or modification
of data.

To enhance
the availability of deployed applications, configure the environment to retry failed
idempotent HTTP requests on all the application server instances serviced by a load
balancer. This option is used for read-only requests, for example, to retry a search
request.

Configure idempotent URLs in the sun-web.xml file. When you
export the load balancer configuration, idempotent URL information is automatically
added to the loadbalancer.xml file.

Upgrading Applications Without Loss of Availability

Upgrading an application to a new version without loss of availability to users
is called a rolling upgrade. Carefully managing the two versions
of the application across the upgrade ensures that current users of the application
complete their tasks without interruption, while new users transparently get the new
version of the application. With a rolling upgrade, users are unaware that the upgrade
occurs.

Application Compatibility

Rolling upgrades pose varying degrees of difficulty depending on the magnitude
of changes between the two application versions.

If the changes are superficial, for example, changes to static text and images,
the two versions of the application are compatible and can both
run at once in the same cluster. Compatible applications must:

Use the same session information

Use compatible database schemas

Have generally compatible application-level business logic

Use the same physical data source

You can perform a rolling upgrade of a compatible application in either a single
cluster or multiple clusters. For more information, see Upgrading In a Single Cluster

If the two versions of an application do not meet all the above criteria, then
the applications are considered incompatible. Executing incompatible
versions of an application in one cluster can corrupt application data and cause session
failover to not function correctly. The problems depend on the type and extent of
the incompatibility. It is good practice to upgrade an incompatible application by
creating a “shadow cluster” to which to deploy the new version and slowly
quiesce the old cluster and application. For more information, see Upgrading Incompatible Applications

The application developer and administrator are the best people to determine
whether application versions are compatible. If in doubt, assume that the versions
are incompatible, since this is the safest approach.

Upgrading In a Single Cluster

You can perform a rolling upgrade of an application deployed to a single cluster,
providing the cluster’s configuration is not shared with any other cluster.

To upgrade an application in a single cluster

Save an old version of the application or back up the domain.

To back up the domain use the asadmin backup-domain command.

Turn off dynamic reconfiguration (if enabled) for the cluster.

To do this with Admin Console:

Expand the Configurations node.

Click the name of the cluster’s configuration.

On the Configuration System Properties page, uncheck the Dynamic Reconfiguration
Enabled box.

If you redeploy using the Admin Console, the domain is automatically
the target. If you use asadmin, specify the target domain. Because dynamic reconfiguration is disabled, the old application continues
to run on the cluster.

Enable the redeployed application for the instances using asadmin
enable-http-lb-application.

For example, for Sun Java System Web Server,
the location is web_server_install_dir/ https-host-name/config/loadbalancer.xml . To ensure that the load balancer loads the new configuration file, be
sure that dynamic reconfiguration is enabled by setting the reloadinterval in the load balancer configuration.

Wait until the timeout has expired.

Monitor the load balancer’s
log file to make sure the instance is offline. If users see a retry URL, skip the
quiescing period and restart the server immediately.

Restart the disabled server instance while the other instances in the
cluster are still running.

Restarting causes the server to synchronize
with the domain and update the application.

Test the application on the restarted server to make sure it runs correctly.

When all server instances have the new application and are running, you
can re-enable dynamic reconfiguration for the cluster again.

Upgrading in Multiple Clusters

To upgrade a compatible application in two or more clusters:

Save an old version of the application or back up the domain.

To back up the domain use the asadmin backup-domain command.

Turn off dynamic reconfiguration (if enabled) for all clusters.

To do this with Admin Console:

Expand the Configurations node.

Click the name of one cluster’s configuration.

On the Configuration System Properties page, uncheck the Dynamic Reconfiguration
Enabled box.

Click Save

Repeat for the other clusters

Alternatively, use this command:

asadmin set --useruser--passwordfilepassword_filecluster_name-config.dynamic-reconfiguration-enabled=false

Redeploy the upgraded application to the target domain.

If you redeploy using the Admin Console, the domain is automatically
the target. If you use asadmin, specify the target domain. Because dynamic reconfiguration is disabled, the old application continues
to run on the clusters.

Enable the redeployed application for the clusters using asadmin
enable-http-lb-application.

For example, for Sun Java System Web Server,
the location is web_server_install_dir/ https-host-name/config/loadbalancer.xml . Dynamic reconfiguration must be enabled for the load balancer (by setting
the reloadinterval in the load balancer configuration), so that
the new load balancer configuration file is loaded automatically.

Wait until the timeout has expired.

Monitor the load balancer’s
log file to make sure the instance is offline. If users see a retry URL, skip the
quiescing period and restart the server immediately.

Restart the disabled cluster while the other clusters are still running.

Restarting causes the cluster to synchronize with the domain and update
the application.

Test the application on the restarted cluster to make sure it runs correctly.

Copy the configuration file to the web server’s configuration directory.

Repeat steps 5 through 8 for the other clusters.

When all server instances have the new application and are running, you
can reenable dynamic reconfiguration for all clusters again.

Upgrading Incompatible Applications

For information on what makes applications compatible, see Application Compatibility the new version of the application is incompatible
with the old. Also, you must upgrade incompatible application in two or more clusters.
If you have only one cluster, create a “shadow cluster” for the upgrade,
as described below.

When upgrading an incompatible application:

Give the new version of the application a different name from the
old version of the application. The steps below assume that the application is renamed.

If the data schemas are incompatible, use different physical data
sources after planning for data migration.

Deploy the new version to a different cluster from the cluster where
the old version is deployed.

Set an appropriately long timeout for the cluster running the old
application before you take it offline, because the requests for the application won’t
fail over to the new cluster. These user sessions will simply fail.

To upgrade an incompatible application by creating a second
cluster

Save an old version of the application or back up the domain.

To back up the domain use the asadmin backup-domain command.

Create a “shadow cluster” on the same or a different set of
machines as the existing cluster.

Use the Admin Console to create the new cluster and reference the existing
cluster’s named configuration.

Customize the ports for the new
instances on each machine to avoid conflict with existing active ports.

For all resources associated with the cluster, add a resource reference
to the newly created cluster using asadmin create-resource-ref.

Create a reference to all other applications deployed to the cluster (except
the current redeployed application) from the newly created cluster using asadmin create-application-ref.

Configure the cluster to be highly available using asadmin configure-ha-cluster.

Create reference to the newly-created cluster in the load balancer configuration
file using asadmin create-http-lb-ref.

Give the new version of application a different name from the old version.

Deploy the new application with the new cluster as the target. Use a different
context root or roots.

Enable the deployed new application for the clusters using asadmin
enable-http-lb-application.

Start the new cluster while the other cluster is still running.

The start causes the cluster to synchronize with the domain and be updated with
the new application.

Test the application on the new cluster to make sure it runs correctly.

Disable the old cluster from the load balancer using asadmin
disable-http-lb-server.

Set a timeout for how long lingering sessions survive.

Enable the new cluster from the load balancer using asadmin enable-http-lb-server.

For example, for Sun Java System Web Server,
the location is web_server_install_dir/ https-host-name/config/loadbalancer.xml . Dynamic reconfiguration must be enabled for the load balancer (by setting
the reloadinterval in the load balancer configuration), so that
the new load balancer configuration file is loaded automatically.

After the timeout period expires or after all users of the old application
have exited, stop the old cluster and delete the old application.

Enabling Load Balancer Logging

Failed-over request information when the request fails over from an
unhealthy instance to a healthy instance.

List of unhealthy instances at the end of every health check cycle.

Note –

When load balancer logging is enabled, and if the web server logging level
is set to DEBUG or to print verbose messages, the load balancer
writes HTTP session IDs in the web server log files. Therefore, if the web server
hosting the load balancer plug-in is in the DMZ, do not use the DEBUG or
similar log level in production environments.

If you must use the DEBUG logging level, turn off load balancer logging by setting require-monitor-data property to false in loadbalancer.xml.

To turn on load balancer logging

Set the logging options in the web server. The procedure depends on the
web server:

With Sun Java System Web Server

In the server’s admin
console, go to the Magnus Editor tab and set the Log
Verbose option to On.

For Apache Web Server, set the log level to DEBUG.

For Microsoft IIS, set the log level to FINE in the sun-passthrough.properties file.

Set the load balancer configuration’s monitor option
to true.

Use the asadmin create-http-lb-config command
to set monitoring to true when you initially create the load balancer configuration,
or use the asadmin set command to set it to true later. Monitoring
is disabled by default.

Understanding Monitoring Messages

The format of the load balancer plug-in log messages is as follows.

The start of an HTTP request contains the following information:

RequestStart Sticky(New) <req-id> <time-stamp> <URL>

The timestamp value is the number of milliseconds from January 1, 1970. For
example: